The Mysterious Benedict Society and the Perilous Journey by Trent Stewart.
The four children who were recruited for The Mysterious Benedict Society come together again under happier circumstances. Their mentor, Mr. Benedict, has promised a "surprise" adventure and not too soon, either. Reynie is happy to be safe with his adopted mother; Sticky loves being safe with his parents; Kate works hard to keep the farm she shares with her newfound father going; and Constance enjoys living with Mr. Benedict - but - regular life is a little boring.
No sooner do the children leave the farm than Kate's father, superspy Milligan, gets a telegram warning the children to stay where they are. Mr. Benedict and his helper, Number Two, have been kidnapped. Too late! The children are already on their adventure
A trip across the ocean in a superfast cargo ship, run-ins with evil Mr. Curtain's henchmen, a seaplane flight to an uncharted island - riddles, packets of money, tunnels...the four youngest members of the Benedict Society are in for a hair-raising adventure.